Does an electric car make economic sense?

I drive 8000 miles a year in a car with 45MPG actual fuel efficiency
(2013 Prius). We paid somewhere around $23000 for it. If I drive this
car for 15 years, I'll buy around 2700 gallons of gas.

Compare this to the (discontinued) Prius Plugin Hybrid with MSRP of
about $30000. Imagine that I could have done all my driving in electric
mode, and that its efficiency is ∞MPGe. I'd break even on the $7000
higher initial price if the average gas price is $2.56. Sounds
plausible that I could save money that way, right?

But of course I couldn't go everywhere in "all-electric" / "charge
depletion" mode, probably only about 2000 miles/year out of my 8000
miles/year would fall into this category (in-city driving 200 days a
year at 10 miles/day). So now I'm only saving only about 670 gallons of
gas *OVER THE COURSE OF OWNING THE CAR FOR 15 YEARS*. This is only
break-even if gas is $10/gallon.

But of course, the plug-in hybrid is not ∞MPGe, it's 110MPGe.
MPGe is actually "miles per 33.7kWh". 33.7kWh of electricity costs $3.17
in my local market at summer rates, or $1.88 at winter rates, an average
of $2.31 (only 4 months are "summer"). So that's $623 in electricity to
operate the thing in electric mode for 30,000 miles. This pushes
the break-even point higher, to around $11/gal gasoline. (Note: a
previous version of this page used incorrect math to arrive at a figure
of $19 as the break-even point)